In clinical practice the diagnosis of vascular diseases such as aneurysms or stenoses is for example essentially based on chronological two-dimensional angiography sequences (in which the blood flow is to be seen). In these examination methods a contrast medium is injected into the bloodstream of a vessel and a sequence of x-ray images is recorded in order to record its propagation over time (bolus front). In addition static three-dimensional volume datasets can be used for diagnosis, which as a rule show a completely filled vessel tree.
Whereas aneurysms mostly show up very clearly in the corresponding images, stenoses are as a rule relatively hard to see. Instead the angiograph shows points in the vessel at which a greatly reduced blood flow is occurring. If a stenosis leads to a complete closure of a vessel the result is that the corresponding vessel, as well as all further vessels supplied by said vessel, are no longer detectable in the x-ray image.
Since the images arising are merely projections of the volume observed, with unfavorable directions of projection overlaying of the vessels occurs in the two-dimensional projections which leads to information loss. In a three-dimensional reconstruction of the blood flow from a 2D angiography sequence this leads to problems since ambiguities can occur during back projection. FIG. 1 gives an example to illustrate this problem of ambiguities. FIG. 1 shows a view A, in which there is back projection from a 2D projection 2D into a 3D volume 3D. A Pixel P is no longer able to be assigned uniquely to a voxel (a number of adjacent voxels lying behind one another) but under some circumstances can be mapped onto a number of voxels V1, V2 belonging to different vessel sections. Even in the 2D projection this demands precise observation to assign of the flow of contrast medium to a vessel. Expressed in general terms the loss of information of the depth information which is caused by the projection makes diagnosis of possible diseases difficult or even makes it impossible to detect said diseases.
To avoid vessel overlays in the 2D projections test fluoroscopy images are prepared at the beginning of each angiography sequence. On the basis of these the doctor maneuvers the detector manually to a suitable position. Subsequently a test image is created again to check the positioning. This process is repeated until such time as an optimum possible view of the entire vessel tree has been found. This means a significantly greater exposure to radiation for the patient than is necessary for the actual angiography sequence. In addition manual alignment is time-intensive since several attempts are needed to establish a suitable patient-detector alignment.
Biplanar angiography systems are a further alternative, in which two views offset by 90° are created for each fluoroscopy step. For very simple vessel overlaying this is a way of reducing ambiguities. For complex vessel structures, even the use of such systems does not allow overlapping of vessels in the projections to be completely avoided.
There are different approaches to dealing with the ambiguities which arise in conjunction with a 3-dimensional blood flow reconstruction. Methods such as those described in [1] and [2] solve the problem with heuristic assumptions and the exclusion of the ambiguous information. In [3], [4] and [5] an approach is already known or has been proposed with which, for each of the ambiguities arising, the probability can be computed of the hypothesis made by the back projection involving correct information or incorrect information.
Basically ambiguities can be reduced by selecting a favorable camera position. Since the bolus front moves however, the determination of a good fixed camera position for the entire sequence of recorded images is possible but a single angle of observation cannot supply optimum information with complicated vessel systems.